Speedometer Off Error Calculation

Basically, if I got from the stock 225/70-15 to a 245/50-16 wheel/tire combo, will my speedo read faster or slower than I am actually going? Someone said there is like a 7% difference in the wheel/tire combo.

A: The 225 is the treadwidth in mm, the 70 is the aspect ratio (% sidwall of treadwidth), and the 15 is the rim diameter. With this, the overall assembly diameter and circumference is:

At 70 MPH with the stock assembly, the rear axle will spin approx 429.45 rpm.

At this same rpm (and same speed on the speedometer), the second setup will actually be going only 65.5 mph while the speedo will indicate 70mph, an error of about 6.8%.

A quicker way to determin the error is to simply ratio the assembly diameters:

1 – (25.6/27.4) = 0.0656 or about 6.6%.

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